1. File Systems
Additional references used: Linux Device Drivers, 3rd ed., O’reilly

2. File Concept Contiguous logical address space Types: Data Program
numeric
character
binary
Program

3. File Structure None - sequence of words, bytes Simple record structure
Lines
Fixed length
Variable length
Complex Structures
Formatted document
Relocatable load file
Can simulate last two with first method by inserting appropriate control characters
Who decides:
Operating system
Program

4. File Attributes Name – only information kept in human-readable form
Identifier – unique tag (number) identifies file within file system
Type – needed for systems that support different types
Location – pointer to file location on device
Size – current file size
Protection – controls who can do reading, writing, executing
Time, date, and user identification – data for protection, security, and usage monitoring
Information about files are kept in the directory structure, which is maintained on the disk

5. File Operations File is an abstract data type Create Write Read
Reposition within file
Delete
Truncate
Open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory
Close (Fi) – move the content of entry Fi in memory to directory structure on disk

6. Open Files
open()
copies entry to Open-file table
Access mode
Several pieces of data are needed to manage open files:
File pointer: pointer to last read/write location, per process that has the file open
File-open count: counter of number of times a file is open – to allow removal of data from open-file table when last processes closes it
Disk location of the file: cache of data access information
Access rights: per-process access mode information
Is open() compulsory for files?

7. Open File Locking What if a file is to be used by many processes?
Per-process open file table
System-wide open file table
Locking provided by some operating systems and file systems
Mediates access to a file
File locks – fcntl() with lock commands
Shared lock (read lock)
Exclusive lock (write lock)
Mandatory or advisory:
Mandatory – access is denied depending on locks held and requested
Exclusive lock by OS by default
Advisory – processes can find status of locks and decide what to do
Programmer must exclusively lock

8. File Types – Name, Extension

9. Access Methods Sequential Access read next write next reset
no read after last write
(rewrite)
Direct Access
read n
write n
position to n
rewrite n
n = relative block number

10. Sequential-access File

11. Simulation of Sequential Access on a Direct-access File

12. Example of Index and Relative Files

13. Directory Structure
A collection of nodes containing information about all files
Directory
Files
F 1
F 2
F 3
F 4
F n
Both the directory structure and the files reside on disk
Backups of these two structures are kept on tapes

14. A Typical File-system Organization

15. Operations Performed on Directory
Search for a file
Create a file
Delete a file
List a directory
Rename a file
Traverse the file system

16. Organize the Directory (Logically) to Obtain
Efficiency
locating a file quickly
Naming
convenient to users
Two users can have same name for different files
The same file can have several different names
Grouping
logical grouping of files by properties, (e.g., all Java programs, all games, …)

17. Single-Level Directory
A single directory for all users
Naming problem
Grouping problem

18. Two-Level Directory Separate directory for each user Path name
Can have the same file name for different user
Efficient searching
No grouping capability

19. Tree-Structured Directories

20. Tree-Structured Directories (Cont)
Efficient searching
Grouping Capability
Current directory (working directory)
cd /spell/mail/prog
type list

21. Tree-Structured Directories (Cont)
Absolute or relative path name
Creating a new file is done in current directory
Delete a file
rm <file-name>
Creating a new subdirectory is done in current directory
mkdir <dir-name>
Example: if in current directory /mail
mkdir count
mail
prog
copy
prt
exp
count
Deleting “mail”  deleting the entire subtree rooted by “mail”

22. Acyclic-Graph Directories
Have shared subdirectories and files

23. Acyclic-Graph Directories (Cont.)
Two different names (aliasing)
If dict deletes list  dangling pointer
Solutions:
Backpointers, so we can delete all pointers Variable size records a problem
Backpointers using a daisy chain organization
Entry-hold-count solution
New directory entry type
Link – another name (pointer) to an existing file
Resolve the link – follow pointer to locate the file

24. General Graph Directory

25. General Graph Directory (Cont.)
How do we guarantee no cycles?
Allow only links to file not subdirectories
Garbage collection
Every time a new link is added use a cycle detection algorithm to determine whether it is OK

26. File System Mounting
A file system must be mounted before it can be accessed
A unmounted file system is mounted at a mount point

27. (a) Existing. (b) Unmounted Partition

28. Mount Point

29. File Sharing – Multiple Users
User IDs identify users, allowing permissions and protections to be per-user
Group IDs allow users to be in groups, permitting group access rights

30. File Sharing – Remote File Systems
Uses networking to allow file system access between systems
Manually via programs like FTP
Automatically, seamlessly using distributed file systems
Semi automatically via the world wide web
Client-server model allows clients to mount remote file systems from servers
Server can serve multiple clients
Client and user-on-client identification is insecure or complicated
NFS is standard UNIX client-server file sharing protocol
CIFS is standard Windows protocol
Standard operating system file calls are translated into remote calls
Distributed Information Systems (distributed naming services) such as LDAP, DNS, NIS, Active Directory implement unified access to information needed for remote computing

31. File Sharing – Consistency Semantics
Consistency semantics specify how multiple users are to access a shared file simultaneously
Similar to process synchronization algorithms
Tend to be less complex due to disk I/O and network latency (for remote file systems
Andrew File System (AFS) implemented complex remote file sharing semantics
Unix file system (UFS) implements:
Writes to an open file visible immediately to other users of the same open file
Sharing file pointer to allow multiple users to read and write concurrently
AFS has session semantics
Writes only visible to sessions starting after the file is closed

32. Protection File owner/creator should be able to control:
what can be done
by whom
Types of access
Read
Write
Execute
Append
Delete
List

33. Access Lists and Groups
Mode of access: read, write, execute
Three classes of users
RWX
a) owner access 7  RWX
b) group access 6  1 1 0
c) public access 1  0 0 1
Ask manager to create a group (unique name), say G, and add some users to the group.
For a particular file (say game) or subdirectory, define an appropriate access.
owner
group
public
chmod
761
game
Attach a group to a file chgrp G game

34. Windows XP Access-control List Management

35. A Sample UNIX Directory Listing

36. File-System Structure
File structure
Logical storage unit
Collection of related information
File system resides on secondary storage (disks)
File system organized into layers
File control block – storage structure consisting of information about a file

37. In-Memory File System Structures

38. Schematic View of Virtual File System
object-oriented way of implementing file systems.
allows the same system call interface to be used for different types of file systems.

39. Allocation Methods
An allocation method refers to how disk blocks are allocated for files:
Contiguous allocation
Linked allocation
Indexed allocation

40. Contiguous Allocation of Disk Space

41. Linked Allocation

42. File-Allocation Table

43. Example of Indexed Allocation

44. Indexed Allocation – Mapping (Cont.)
outer-index
index table
file

45. Combined Scheme: UNIX (4K bytes per block)

46. Free Space Management Bit Vector …  1 2 n-1 0  block[i] free1 2
n-1
0  block[i] free
1  block[i] occupied
bit[i] =

Block number calculation
(number of bits per word) *
(number of 0-value words) +
offset of first 1 bit

47. Free Space Management(contd)
Linked List
Separate list for list of free nodes
Full use of all available blocks
Complexity
overhead
Cluster allocation
Grouping
Store n free blocks together.
Counting
Maintain free block list as (startblock,count) pair
Good for contiguous free blocks
Bad if ??

48. Unix File System File data is stream of bytes
Upto program to associate special meaning to files
Directories interpreted by OS/programs
Access Permissions

50. File system Layout : UNIX
User File Descriptor Table
File Table
Inode Table
Inode  index node
Boot block
Super block
Inode-list
Data blocks

51. File System Algorithms
namei
alloc free
ialloc ifree
iget iput bmap
Buffer allocation algorithms
getblk brelease bread breada bwrite

52. inode File owner identifier File type File access permissions
File access time details
Number of links to the file
Table for disk addresses of data
File size
In-memory
Status
Inode lock status
Process waiting for inode
Mount point??
Logival device number
Inode number
Position of inode in on disk inode-list
Pointers to other inodes
Links to hash-queue
Free list
Reference count (how many opens)

54. Buffer In-memory copy of disk blocks Auxiliary data
memory array
Storing disk blocks
Buffer header
Buffer info
Auxiliary data
Superblock
Inode-list
System response time
Throughput
Pre-cache data
Delay-write data to disk
Locked/unlocked
Valid Data
Delayed-write
In-use
Waiting process

55. Buffer Pool
Block identified by logical device number-block number pair
Free list
LRU
Cirucular doubly-linked list of free buffers
Buffer added to free list at back
(on errors may be to front too)
Buffer removed on request from front
(may be middle if block in free list)
Buffer header
Hashed on logical device number-block number pair

56. getblk
Scenario 1:
Buffer already in free list

57. Scenario 3 is shown in right Scenario 4
What if no free buffer available and block not in hash queue
Scenario 5
Block in queue, but locked by some other process

58. Block release - brelse
Wakeup all procs, events waiting for any buffer to be free
Wakeup all procs, events waiting for this buffer to be free
If buffer is valid and not old
Enqueue to back of free list
Else
Add to front of free list
Unlock buffer
Inside synchronization construct

59. Block read
Block read

60. Block Write

61. Inode Allocation

63. ialloc(contd)

64. ifree

65. Requesting and Freeing Disk Blocks

67. File System Calls

68. Open

69. After Close is called in process B
After opening of files by processes A and B

71. Write and Lseek Write Lseek Inverse of read.
If no block available for writing, get a new block and continue writing.
All writes back to disk are lazy writing.
Very short-lived files like temporary files in /tmp may reside only in memory and never reach disk
Lseek
Just modifies the file offset value in file table

73. Special files Char file Block special files Representative of device.
For referencing the actual device.
Major number
Minor number

77. Modifications in iget

78. Modifications in namei

80. Links Make file/directory point to same inode
Directory linking done only by super-user to ensure no infinite loops.(is root good?)
implementation very similar to mkdir

82. Virtual File System Layer
Open,…
System call Interface …
file = fget_light(fd, &fput_needed);
if (file) { …
ret = vfs_read(file, buf, count, &pos);
…//update offset and rest }
vfs_read(…)
{ …
if (file->f_op->read)
ret = file->f_op->read(file, buf, count, pos);
… }
File System
Read  fs/readwrite.c
Open  fs/open.c …
Virtual File System Layer
ext2
ext3 …
Buffer
Device Drivers

83. Ext2 File table, fd table File system stuff include/linux/file.h
struct file
struct fdtable
File system stuff
include/linux/fs.h
include/linux/<spcific>_fs.h
include/linux/ext3_fs.h